Agree with the other posters, the lower the ratio, the better. Not all the babies in daycare will be six months old. Others will be crawling or walking, and will need more attention. The more kids in daycare, the less time the caregiver will have to devote to your baby, especially if your baby is more easy going/less mobile.
From a developmental perspective, at six months old, they are starting to be more able to interact with the world, and picking up more language. Also should be sitting up, picking things up, and continuing to develop social responses (joint attention, social smiling, etc.) More available staff facilitates increased time spent individually interacting with children (though I realize that this may not be true in every single case).
But at the end of the day, it really comes down to the feel you get after you interview the teachers and observe how they manage the children. I've dropped by unannounced several times at my DD's daycare and sometimes spent a couple hours just sitting in the corner observing the lesson and activities and how the teachers provide structure, discipline, and engage the children. And I also watch how DD reacts when I announce we are going to "school". She still gets very excited and can't wait to get out of her stroller when we arrive at the daycare, so that's a good sign, as she has a completely opposite reaction and heads for the exit whenever we enter the doctor's office where she gets her immunizations.